The present invention relates to the isomerisation of the Z-isomer I-Z of semicarbazone compounds of the general formula I into its E-isomer I-E
where the variables in formula I have the following meanings:    m, p and q are each independently an integer of 0, 1, 2, 3 or 4    R1, R2, R3 are each independently halogen; OH; CN; NO2;            C1-C6-alkyl, optionally substituted with C1-C4-alkoxy, C1-C4-haloalkoxy or C3-C6-cycloalkyl;        C1-C6-haloalkyl;        C3-C6-cycloalkyl;        C1-C6-alkoxy, optionally substituted with C1-C4-alkoxy or C3-C6-cycloalkyl;        C1-C6-haloalkoxy;        C1-C6-alkylcarbonyl;        C3-C6-cycloalkoxy;        C1-C6-alkoxycarbonyl or        C1-C6-alkoxycarbonyloxy.        
Semicarbazone compounds of the general formula I are known from EP-A-462456 to be effective as pest-controlling agents. Semicarbazones of the formula I have two geometrical isomers with regard to the C═N-double bond, namely the E-form I-E and the Z-form I-Z.

At room temperature these geometrical isomers are stable with regard to E/Z-isomerisation. As regards the relative pesticidal activity of these compounds, the E-form I-E is generally more active than the Z-form I-Z. Therefore, agriculturally and commercially acceptable specifications of semicarbazones I require an E/Z-ratio of at least 9:1 and preferably at least 10:1.
Compounds of the formula I can be prepared by the process illustrated in the following scheme:

Significant amounts of the undesired Z-isomer I-Z are formed by this process. Moreover, much effort is needed to achieve the desired E/Z-ratio. Firstly, long reaction times are required to achieve a high E/Z-ratio in the hydrazone precursor II, necessary for obtaining the desired E/Z-ratio in the final product I. Secondly, the crystallisation of the E-isomer I-E in the presence of the Z-isomer I-Z is tedious and difficult. In order to obtain a high isolated yield of the desired E-isomer, some of the Z-isomer must also be crystallized with the E-isomer from the reaction mixture. Similarly, in order to obtain the desired E/Z-ratio in the crystallized product, a low isolated yield of the E-isomer is necessary, so that the undesired Z-isomer is completely solubilised along with significant amount of E-isomer in the reaction mixture. Thirdly, recrystallisation of isolated product I containing significant amounts of the undesired Z-isomer to obtain the desired E/Z-ratio is also tedious and difficult. As with crystallisation from the reaction mixture, either low crystallisation recoveries or high Z-isomer content of the final product are obtained.
These involve the risk of either isolating a product in low yield or not having not required E/Z-ratio.
Consequently, there is a need for a method which allows a simple isomerisation of the Z-isomer of I into its E-isomer I-E.